【作者】 周明；

【导师】 陈军； 李细锋；

【作者基本信息】 上海交通大学 ， 材料科学与工程， 2018， 硕士

【摘要】 汽车轻量化设计需求使高强度钢板在车身结构上得到广泛应用,但钢板高强度化的同时也给其成形过程带来了挑战。高强度钢板在冲压成形过程中,不可避免地会使用冲裁等分离工序,然而剪切分离过程造成的边部缺陷很容易成为后续成形过程中的裂纹源,使材料在较低应变状态下发生边部开裂,严重降低材料的成形性能,进而影响生产的稳定性。为探索高强钢高质量冲切和无缺陷边部成形工艺,在国家重点研发计划项目(编号:2017YFB0304403)的资助下,本文采用边部冲裁实验和基于数字图像相关技术(Digital Image Correlation,DIC)的剪切边拉伸试验,系统地研究了冲裁工艺参数对TRIP780和QP980冲切质量及后续拉伸特性的影响。另外,对激光切割、冲裁和线切割等代表性切割工艺进行了对比研究。基于DIC技术,对不同边部质量试样的拉伸断裂过程进行追踪,并采用扫描电镜对断口形貌进行分析,揭示了不同边部质量下高强度钢板的开裂模式及微观机理。取得了如下主要研究成果:(1)实验研究了冲裁工艺参数对TRIP780和QP980冲切断面质量及后续拉伸特性的影响。结果表明:两种材料随着冲裁间隙的增大,冲切断面质量和拉伸性能均逐渐下降;在8.25mm/s~57.75mm/s的速度研究范围内,冲裁速度影响较小;采用5°斜刃冲裁,断面中光亮带和毛刺比例同时减小,后续拉伸性能显著增加;继续增大刃口角度,断面质量和拉伸性能提升较小。(2)对比研究了激光切割、线切割、冲裁和冲裁+修整几种典型切割工艺对QP980切割边成形能力的影响。研究发现:0°平刃冲裁试样的延伸率和断裂应变最低,其次是激光切割试样,10°斜刃冲裁、0°平刃冲裁+打磨和线切割加工试样的延伸率和断裂应变最高;相比而言,斜刃冲裁是一种既能保证切割质量又适合大批量生产的切割工艺;通过对比平刃和斜刃冲裁断面的宏、微观形貌,发现斜刃冲裁断面毛刺较小、边部整齐、断裂面空穴损伤小,使得斜刃冲裁边拥有优异的拉伸性能。(3)不同边部冲切质量的试样在拉伸过程中主要呈现如下2种断裂形式:颈缩型断裂和边部开裂型断裂。基于DIC技术对拉伸断裂过程进行追踪,揭示了2种断裂形式下裂纹的演化过程,同时结合扫描电镜对断口进行宏、微观形貌分析,揭示了断裂的微观机理。更多还原

【Abstract】 The lightweight design demand of automobile has made high strength steel widely used in car body structures,but the higher strength of steel also brings more challenges in its forming process.During the stamping process of high strength steel,it is inevitable to use blanking or other cutting processes.However,the edge defects caused by shearing process easily become the source of cracks in subsequent forming process,which makes the material crack at the sheared edge under very low strain state and severely reduces the formability of the material.As a result,the stability of production is affected.With the support from National Key Research and Development Project(No.2017YFB0304403),high-quality shearing and non-defective edge forming processes for high strength steel were explored in this paper.The edge blanking test and sheared edge tension test based on digital image correlation(DIC)technology were used to investigate the influences of blanking parameters on sheared edge quality and subsequently tensile property of TRIP780 and QP980 steels.In addition,a comparative study was conducted on the representative cutting techniques such as laser cutting,blanking and wire electrical discharge machining(Wire-EDM).Based on the tracking of the fracture process with DIC system and the analysis of fracture surface morphology,the fracture modes and micro-mechanisms of high strength steel with different edge qualities were also revealed.The major achievements obtained are as follows:(1)The effects of blanking parameters on sheared edge quality and subsequently tensile property of TRIP780 and QP980 were studied experimentally.The results show that for both materials,as the blanking clearance increases,the sheared edge quality and tensile performance gradually decrease.The blanking speed has few effects when velocity value varies between 8.25mm/s and 57.75mm/s.When using 5° oblique edge blanking,the ratio of burnish zone and burr zone decreases simultaneously,and the subsequently tensile performance increases significantly.With the further increase of punch edge angle,the enhancement of sheared edge quality and tensile performance is modest.(2)The influence of several typical cutting processes such as laser cutting,Wire-EDM,blanking and blanking with polishing on cut edge tensile property of QP980 were comparative studied.The results show that the level blade blanked specimens have the lowest elongation and fracture strain,followed by laser-cut specimens.The specimens cut by 10° oblique edge blanking,Wire-EDM,level blade blanking with polishing have the highest elongation and fracture strain.However,in contrast,oblique edge blanking can not only guarantee the cutting quality,but also be suitable for mass production.Oblique edge blanked surface has less void damage,meanwhile,the burr height is small and the edge is smooth,which makes the sheared edge have superior tensile properties.(3)Two typical fracture modes were found in sheared edge tension test for specimens with different edge qualities.They are fracture by necking and fracture by edge cracking.In this paper,crack evolution process of above two fracture modes was revealed based on the tracking of the fracture process with DIC system.The microstructure of the fracture surface was analyzed by scanning electron microscope(SEM),and the microcosmic mechanism of the two types of fracture was analyzed.更多还原